The invention relates to an optical transmission system comprising a radiation source, a monomode optical transmission fibre having an end portion in which the core diameter varies, and a lens system interposed between the radiation source and the transmission fibre.
A transmission fibre having an end portion in which the core diameter varies is disclosed in European Patent Application No. 0,114,439. Said application describes a monomode optical transmission fibre whose end portion has a tapered core and cladding. The end of the fibre carries a spherical lens of a material having a refractive index which is substantially higher than the refractive index of the core material of the fibre.
A transmission fibre having a tapered core and cladding has suitable properties with respect to the permissible offset of the fibre relative to the radiation source, for example, a diode laser, and the feedback of radiation to the radiation source as a result of reflections within the fibre. Such a fibre has a comparatively high coupling efficiency. The coupling efficiency is that fraction of the radiation emitted by the radiation source which is transmitted in the fibre. The presence of a lens on the tapered end portion of the fibre, for example, a lens having a high refractive index and consequently a large numerical aperture, leads to a further increase in coupling efficiency.
It may be desirable to increase the coupling efficiency even further because this enables the number of intermediate amplifiers in a long-distance transmission line to be reduced or because it enables weaker radiation sources, for example light-emitting diodes, to be used, which diodes are of a simpler construction and consequently cheaper. It has been found that a part of the residual coupling loss is due to a suboptimum matching of the wavefront issuing from the lens to the wavefront entering the fibre.